The present invention relates to an electrical operating means, and more particularly, to a gear transmission for an electrical gear box adapted to be used for opening and closing valve means.
In slice valves and globe valves, several or more than ten manual rotations of a handle is required to open and close these valves, whereas in butterfly valves and ball valves, onlya quarter turn of the valve shaft is required for opening and closing the valves, which is extremely convenient. However, in these butterfly or ball valves, since the opening and closing operation of the valves can be immediately achieved, the flowing condition of fluid or fluid pressure is immediately changed resulting in disadvantageous water hammer phenomena occurring in a fluid passage. Therefore, it is necessary to reduce the opening and closing speed of the valve by an electrical operating means at a reduction ratio range of 1/1000 to 1/4000. The reduction ratio is 20 to 100 times larger than that required for a slice valve or globe valve.
FIG. 1 shows a conventional gear transmission including a gear box 5 and which is adapted to be used in an electrical operator for decelerating transmitting rotation of a motor shaft to an output shaft. Six stages of spur gers are provided to obtain a reduction ratio of 1/2000. The rotation of a motor 1 is successively decelerated from first stage gears 2a and 3a through sixth stage gears 2f and 3f to operate the valve by an finally decelerated output shaft C connected to a valve shaft by means of a coupler (not shown).
Gear module number as defined by Japanese Industrial Standard JIS B1701 is determined by the following formula: EQU M=d/z
wherein
M: module number in mm PA1 D: diameter of standard pitch circle in mm PA1 Z: number of gear teeth
The module number of the gears 2a and 3a of the first stage is in a range of 0.4 to 1 and the teeth width thereof is in a range of 2 to 4 mm. The module of the gears 2f and 3f of the sixth stage is in a range of 1.5 to 3 and the teeth width thereof is in a range of 15 to 30 mm. The diameter of each of gear shaft 4a to 4e and C is increased in turn.
With this structure, since there are six shafts in the gear box 5, the size of the box is large in comparison with the motor housing 1, so that the total size of the operation means becomes large, which is troublesome for installing the operation means in actual piping work, particularly above the valve portion. In order to provide an anti-explosion type operating means, the gear box is desirably cylindrical to ensure the necessary clearances, for example, a clearance between the motor shaft and a hole formed in the upper supporting plate to receive the shaft. The cylindrical gear box is also desirable in view of the size of bearings, threaded holes for securing the motor and the relationship of fixing and supporting upper and lower plates of the gear box. However, according to the conventional device, the shape of the box is required to be rectangular in order to provide the most compact structure, since generally a large space is required if a cylindrical box is proposed.